Cenozoic Evolution of Larger Benthic Foraminifers: Paleoceanographic Evidence for Changing Habitats
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Proceedings of the 11th International Coral Reef Symposium, Ft. Lauderdale, Florida, 7-11 July 2008 Session number 1 Cenozoic Evolution of Larger Benthic Foraminifers: Paleoceanographic Evidence for Changing Habitats P. Hallock1, L. Pomar2 1) University of South Florida, College of Marine Science, 140 7th Avenue S., St. Petersburg, FL 33701, USA 2) Universitat de les Illes Balears, Departament de Ciencies de la Terra, E-07122 Palma de Mallorca, Spain Abstract. The ever-increasing treasure-trove of paleoceanographic data relating to evolving Cenozoic ocean structure, including geographic and bathymetric gradients, provides novel insights into long-term changes in environmental conditions influencing shelf, ramp and oceanic-platform habitats occupied by carbonate- producing ecosystems. Similarly, recent studies documenting the influence of internal waves on mid- and deep- shelf habitats provides equally exciting insights into previously unrecognized environmental variability experienced by organisms living in those habitats. Paleocene-Eocene photic-dependent carbonates were dominated by calcitic coralline red algae and larger benthic foraminifers (LBF), with aragonitic corals and calcareous green algae more restricted temporally and spatially. Morphologies of LBF are strongly influenced by light availability and water motion, with larger, flatter and more fragile shapes characteristic of lower light, low wave-energy environments. Since substantial LBF habitat is at middle to outer shelf or ramp depths (i.e., ~30 to ~130 m), understanding the influence of internal waves on these habitats as oceanic thermal gradients developed through the Cenozoic can provide crucial insights into evolving environmental conditions where the most diverse and highly specialized LBF biotas occurred. Key words: Carbonate ramp, reef, internal waves, thermocline gradients, symbiosis Introduction influence of internal waves at those depths (e.g., Understanding biological and geochemical processes Wolanski et al. 2004; Leichter et al. 2005) with the associated with modern carbonate systems is essential records of oceanic thermal gradients through the to interpreting fossil reefs and carbonate Cenozoic (e.g., Dutton et al. 2005) can provide crucial sedimentation. Recognizing the limitations of insights into how environmental conditions changed uniformitarianism is equally crucial (Pomar & for middle and outer shelf benthic habitats. Hallock 2008). Cenozoic carbonate-producing ecosystems emerged from the remnants of Cretaceous Larger Benthic Foraminifers (LBF) biotas, evolving in the warm alkaline oceans of a The LBF are an informal group of benthic Greenhouse world, then modifying as Icehouse foraminifers characterized by relative large size (e.g., conditions developed. The latter included stronger generally > 1mm and up to 6 cm in diameter – Lee latitudinal and bathymetric temperature gradients, 1998) and complex internal morphologies. Modern declining atmospheric CO2 concentrations and representatives host a rich variety of algal declining calcium concentrations and alkalinity in the endosymbionts in a relationship similar to that oceans. between corals and their zooxanthellae. By analogy, Paleocene-Eocene photic-dependent carbonates fossil LBF are interpreted to also have hosted algal were dominated by calcitic coralline red algae and symbionts (e.g., Lee 1998; Hallock 1999; and larger benthic foraminifers (LBF), with aragonitic references therein). corals and calcareous green algae more restricted Extant LBF worldwide belong to members of six temporally and spatially. In this paper, we compare long-ranging families, the Amphisteginidae, Hottinger’s (1998) synthesis of records of Alveolinidae, Nummulitidae, Peneroplidae, Rotaliidae, diversifications and extinctions of Cenozoic LBF and Soritidae (e.g., Hallock 1999). One other family, lineages in the context of published evidence for the Calcarinidae, is characteristic of the late Neogene changes in ocean circulation and thermocline in the Indo-west Pacific, where some species are structure as Icehouse conditions developed (e.g., Lear prolific producers of beach sands (e.g., Hohenegger et al. 2000). Moreover, given that much of the 2006). Amphisteginidae, Calcarinidae, Nummulitidae diversification was at middle to outer shelf depths (i.e., and Rotaliidae are all families in the order Rotaliida, ~30 to ~130 m), synthesizing emerging records of the and are characterized by relatively transparent (i.e., 16 hyaline) calcite shells. Members of these families transparency only occurs where surface waters are typically host diatom endosymbionts. In contrast, the sediment free and have extremely low nutrient and other families are Miliolida, whose shells of randomly plankton concentrations. arranged calcite crystals covered by a veneer of brick- LBF assemblages typically characterize Pomar’s like crystals impart a porcelaneous appearance that is (2001) light-defined zones (Fig. 1). Most of the relatively opaque to light (e.g., Debenay et al. 2000). miliolid LBF, as well as most calcarinids, live The shells of many of the larger miliolids have primarily in euphotic habitats. On the Florida reef thinned outer walls to permit light into the tract, Soritidae (especially the Archaiasinae) show chamberlets. Larger miliolids host a diversity of algal depth zonation, but none are common below about 30 symbionts (e.g., Lee & Anderson 1991). The m. Worldwide, robust Amphistegina also are found Alveolinidae host diatoms. One lineage of abundantly at less than 30 m, with progressively Peneroplidae hosts rhodophyte symbionts while a thinner or smaller chambered morphologies second lineage has chlorophyte symbionts. Similarly, dominating below 30 m. Nummulitids can be found one lineage in the Soritidae hosts dinoflagellate at less than 30 m, but most dominate at mesophotic to symbionts while the second hosts chlorophytes. oligophotic depths (>30m, see Hohenegger 2005). Distributions of modern taxa of LBF are strongly Several extant nummulite species, whose shapes are influenced by light (Fig.1). Both between species and very thin and flat, have depth distributions that peak within species trends in morphology, especially shape at 70 m or deeper. and surface-to-volume ratios, reflect light intensity Water motion is another important environmental and water motion (e.g., Hohenegger 2005), both of parameter influencing LBF distributions. Both wave which influence rates of calcification (e.g., Hallock et energy and light decrease exponentially with depth, al. 1986). Thus, both conceptual (Hallock & Glenn and both influence shell morphologies similarly, 1986; Beavington-Penney & Racey 2004) and which is one reason LBF are excellent paleodepth numerical (e.g., Hallock 1987; Mateu-Vicens et al. indicators. Lenticular shapes, thicker shell walls and, 2008) models have been developed to interpret in the case of the calcarinids, stellate morphologies paleoenvironments of LBF-rich limestones. allow such foraminifers to thrive and hypercalcify in Hallock (1988, 1999) also noted that some taxa, high light, high wave-energy environments such as notably the porcelaneous Soritidae and the stellate reef flats of the Indo-Pacific. Calcarinidae, have tended to diversify “laterally”, specializing to different relatively shallow-water, higher light habitats, while modern Amphisteginidae, and Nummulitidae have diversified vertically, with robust shapes characteristic of shallower dwelling species and flat shapes characteristic of deeper depths. Historically, the terms photic or euphotic have been used to describe environments where there was sufficient light for an excess of photosynthesis over respiration (e.g., Hallock & Schlager 1986). Pomar (2001) proposed terminology to distinguish among Figure 1: Major families of larger benthic foraminifers illustrating photic habitats. He used “euphotic” only for depths shapes and general depth ranges (Hallock 1999) in the context of euphotic, mesophotic and oligophotic habitats as defined by Pomar where light is sufficient for high rates of (2001). photosynthesis and associated hypercalcification, typically less than 30 m even in very clear water. He In contrast, discoid morphologies are best adapted used “mesophotic” to characterize depths where light to quieter environments, where they either burrow is sufficient for photosynthesis to support substantial their apertural face into the surficial sediments at a calcification rates, but not true hypercalcification, i.e., low angle, leaving most of the dorsal surface exposed depths in the 20-70 m range, again dependent upon to light, or sit flat on the substratum. Either way, they water transparency. He used the term “oligophotic” to are superbly adapted to harvesting food and dissolved characterize depths where there is limited light nutrients from the substratum and pore waters, while penetration sufficient to support calcifiers like their algal symbionts capture radiant energy from coralline red algae, and very thin flat LBF and corals. sunlight. If their habitat is infrequently disturbed, e.g., As Hallock (1987) demonstrated using a modeling by storm waves or deposit-feeding animals, these approach, well developed biotas adapted to low light foraminifers can literally spend years accumulating for photosynthesis depend upon consistently high the sparse resources required for reproduction water transparency that permits light penetration to (Hallock 1985). Exceptions to this generalization for depths >70 m. Such consistently high water discoid morphologies